The present invention relates to a hologram recording apparatus and method that records data by holography, and to a hologram reproducing apparatus and method that reproduces data recorded by holography.
In holographic data recording, two laser beams, a modulated (data superimposed) signal light beam and an unmodulated reference light beam, are generated from laser light, and are applied onto the same position on a hologram recording medium. Consequently, the signal light beam and the reference light beam interfere with each other on the hologram recording medium, and a diffraction grating (hologram) is thereby formed at the irradiated position and is recorded as data on the hologram recording medium.
Some hologram recording and reproducing apparatuses record data, for example, by using both red laser light and blue laser light. Red laser light is used to read an address servo pattern prerecorded in the form of pits on the recording medium and to access a target address according to the read pattern. Blue laser light is used to record data on a hologram recording layer different from a layer in which the address servo pattern is recorded.
In hologram recording and reproducing apparatuses, it is required that the position and angle of a beam be fixed during recording or reproduction, and that reproducibility thereof be high. In a disc-type recording and reproducing apparatus, recording or reproduction is typically performed while a disc serving as a recording medium is being rotated at a constant velocity. For example, the rotation of an optical disc is not started or stopped every time one pit is recorded thereon. This is because rapid acceleration or deceleration of the disc increases the inertia of the disc, and results in excessive energy consumption.
This apparatus is required to record or reproduce a hologram within a short time. Since a disc serving as a hologram recording medium slightly rotates even during recording or reproduction, when recording or reproduction continues beyond a predetermined time, the position of a beam spot of laser light on the disc is not satisfactorily fixed.
There are the following limitations on reduction of the recording and reproduction time. A certain amount of light power is necessary to expose a recording medium to light. In order to increase the laser power for that purpose, a higher-power laser element and more electric power are necessary. Therefore, from the viewpoint of recording efficiency, it is preferable that the required laser power be as low as possible, and as a result, it is preferable that the exposure time be as long as possible. During reproduction, a highly sensitive light receiving element is necessary to obtain the required reproduction S/N ratio in a short observation time. Therefore, in order to optimize the S/N ratio, it is preferable that the laser power applied to the light receiving element be large, or the exposure time be long, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-85768.
In order to satisfy these opposite conditions, Japanese Unexamined Patent Application Publication No. 2003-178458 discloses an apparatus in which the relative velocity between a recording and reproducing beam and a disc is zero or considerably close to zero while the beam is reciprocatingly oscillated in the same direction as the rotating direction of the disc.
However, in the apparatus disclosed in the latter publication, since an objective lens is oscillated in the rotating direction of the disc by an actuator, for example, when two laser light beams, red laser light and blue laser light, are used, as described above, both the laser light beams are oscillated. As a result, it is difficult to read the address with the red laser light, and to synchronize the rotation of the disc and recording or reproducing operation.
When the time for high-speed recording and reproduction, that is, the time t taken from when an operation of recording (or reproducing) one hologram until when an operation of recording (or reproducing) the next hologram decreases, that is, 1/t=f (Hz) increases, it is difficult to properly control the reciprocating motion of the objective lens. In this case, f represents the reciprocation frequency. In order to achieve desired recording density and transfer rate in actual products, for example, it is necessary to write multiple holograms so that the holograms are shifted from one another by 10 to 20 μm when f=1 kHz. In this case, even if the deviation between the beam application position and the target recording position on the disc can be obtained according to signals indicating information about the absolute position on the disc in the rotating circumferential direction, it may be actually impossible for the beam to follow the target recording position with high precision. Even if the beam can follow the target recording position, when the recording frequency is high, as described above, an unnecessarily high accuracy in measuring the deviation from the target position, a high sample frequency, a wide servo control region, and an actuator having a wide piston motion range are necessary. This is not practical.
It is desirable to provide a hologram recording apparatus, a hologram reproducing apparatus, a hologram recording method, and a hologram reproducing method that can perform recording and reproduction with as long exposure time as possible even when two types of laser light beams are used.
It is also desirable to provide a hologram recording apparatus and a hologram recording method that allow reliable recording at a desired recording position even when the reciprocation frequency is high. It is also desirable to provide a hologram reproducing apparatus and a hologram reproducing method that can reproduce signals, which are recorded by the above apparatus and method, with a high S/N ratio.